1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, for example, a transistor (a thin film transistor and the like) can be given. In addition, a semiconductor device also refers to a display device such as an EL display device.
2. Description of the Related Art
A technique by which a transistor is formed using a semiconductor thin film formed over a substrate has attracted attention. The transistor is applied to a wide range of electronic devices such as a liquid crystal display device. A silicon-based semiconductor material has been widely known as a material for a semiconductor thin film applicable to a transistor. Besides, an oxide semiconductor has attracted attention.
FIG. 13 is a top view of a bottom-gate transistor in which In—Ga—Zn—O (IGZO) is used as an oxide semiconductor. This transistor includes a gate insulating film (not shown) formed over a gate electrode 1001, an oxide semiconductor film 1002 which is formed using IGZO over the gate insulating film, and a source electrode 1003 and a drain electrode 1004 which are formed over the oxide semiconductor film 1002 (for example, see Patent Document 1).
FIG. 14 shows Id (drain current)−Vg (gate voltage) curves of the transistor shown in FIG. 13 which are obtained before and after GBT (Gate Bias Temperature) stress test. The broken line shows the Id−Vg curve before the GBT stress test and the solid line shows the Id−Vg curve after the GBT stress test.
In the transistor shown in FIG. 13, because an entire end region of the oxide semiconductor film 1002 overlaps with the gate electrode 1001, stress is generated in the end region of the oxide semiconductor film 1002 due to an electric field by application of negative voltage to the gate electrode 1001 when the GBT stress test is performed. Thus, the end region of the oxide semiconductor film 1002 is easily made to have n-type conductivity. Accordingly, a parasitic channel is generated in the end region of the oxide semiconductor film 1002 and a leakage path is formed between the source electrode 1003 and the drain electrode 1004, which results in a hump in the Id−Vg curve as shown in FIG. 14. In the circuit using the above transistor, the generation of the hump has adverse effects that voltage in the circuit cannot be retained, current consumption is increased, and the like. Note that “hump” means that the Id increases in the Id−Vg curve in stages.
The reason that the end region of the oxide semiconductor film 1002 becomes n-type is as follows. The oxide semiconductor film 1002 is activated by generation of electric field stress in the oxide semiconductor film 1002, and its reactivity is enhanced. In particular, when the oxide semiconductor film 1002 is formed using a photolithography process and an etching process in manufacturing the transistor, a side surface of the end region of the oxide semiconductor film 1002 is directly exposed to an etching atmosphere such as plasma although a top surface of the oxide semiconductor film 1002 is protected by a photoresist layer. Thus, the side surface of the oxide semiconductor film 1002 is damaged in the process and is easily contaminated by an impurity. As a result, the end region of the oxide semiconductor film 1002 is more easily activated compared to the other portion of the oxide semiconductor film 1002, thereby being likely to become n-type by the GBT stress test.